An optical disc has been developed from a CD (compact disc) and a DVD (digital versatile disc) to a next generation high-density disc, and the recording density or packing density of the optical disc has been increasing. Data on the optical disc is recorded by, for example, forming very fine patterns or grooves on a surface of a substrate made from polycarbonate, and the recorded data is read by scanning the patterns or grooves with a laser beam. In order to improve the recording density, an optical disc with multiple recording surfaces on a single disc has been produced. Such an optical disc is produced by adhering two or more disc substrates having a recording surface by using a resin as an adhesive.
In a case of adhering two or more disc substrates by using the resin as the adhesive, in general, first, the adhesive is supplied on an area close to the center of one of the disc substrates so as to be a circular shape, second, the disc substrates are being spun after combination with other disc substrates, and third, the resin between the disc substrates is spun off by extending the excessive adhesive in order to obtain a film of the resin which is evenly spread on an overall surface of the disc substrate with a uniform thickness, After the above-described steps, the optical disc including two or more combined disc substrates is transported to an ultraviolet radiation apparatus in order to cure the overall extended adhesive.
Regarding a single disc substrate before combination with other disc substrates, it is known that a warp or a curve may be caused when the substrate is formed by using a metal mold and when the disc substrate is taken out of the metal mold, a warp or a curve may be caused in a step of transporting the disc substrate by using a transportation mechanism, and the warp or curve of the disc substrate may be maintained by curing or hardening the disc substrate in such a state. If the disc substrates which respectively have a warp or curve are combined, the disc substrate obtained by combining such disc substrates has a warp or curve.
When the adhesive between the combined disc substrates is cured, because of the influence of heat caused by light radiated from an ultraviolet radiation apparatus, and because of the influence of such differences of transmittance caused by differences of materials and/or constitution of films formed on the disc substrate, there is a possibility of causing a temperature difference between an upper side substrate of the combined disc substrates on which the light is radiated and a lower side substrate of the combined disc substrates which touches a mounting table. In addition, both a heat included in the radiated light and the heat of reaction of the adhesive which is an ultraviolet curable resin is conducted to a mounting table, a temperature of the mounting table is raised, and consequently, a temperature difference can be caused between an upper side substrate of the combined disc substrates on which the light is radiated and a lower side substrate of the combined disc substrates which touches the mounting table. In general, an upper side substrate is largely extended by the influence of a large amount of heat compared to a lower side substrate which is less extended by the influence of the small amount of heat. If the adhesive is cured in such a state, after cooling of the substrate, the upper side substrate has a larger shrinkage compared to the lower side substrate, and the overall disc substrate is in a curved shape dipping at a center portion.
There is a generally-known technique for controlling such a curve or warp, that is disclosed in, for example, Patent Document 1. In Patent Document 1, a mounting table on which the substrate is mounted provides a cooling portion for preventing the mounting table from increasing the temperature and keeping the mounting table at a predetermined temperature, and the temperature difference between an upper side substrate and a lower side substrate is controlled to be small in order to prevent the substrate from having a warp or curve. On the other hand, in Patent Document 2, the amount of warp or curve of the substrate after radiating ultraviolet light is measured beforehand, a temperature difference between the upper side substrate and the lower side substrate is caused by a heating means based on the measured amount of warp or curve before radiating ultraviolet light, and consequently, the warp or curve of the substrate is controlled.
In accordance with such conventional control methods using phenomena of being curved by heat, it is possible to control the overall shape of the substrate so as to be raising or dipping with an axis or center that is the central aperture of the substrate. However, it is difficult to control a partial warp or curve by controlling the temperature of a corresponding portion because the heat is conducted in the substrate. In particular, in a case in which the substrate includes a partial warp or curve at the outside edges after molding, there is a possibility of including a warp or curve in the combined substrate at the outside edge, and it has been desired to reduce such a partial warp or curve. The present invention has been conceived in order to solve such a problem and has an objective to automatically control the warp or curve to be in a desirable state.    [Patent Document 1] Japanese Patent Application, First Publication No. 2003-99985    [Patent Document 2] Japanese Patent Application, First Publication No. 2002-251803